Method and apparatus providing final test and trimming for a power supply controller

ABSTRACT

A power supply controller having final test and trim circuitry. In one embodiment, a power supply controller for switched mode power supply includes a selector circuit, a trim circuit, a shutdown circuit and a disable circuit. The trim circuit includes a programmable circuit connection that can be selected by the selector circuit by toggling a voltage on an external terminal such as for example a power supply terminal, a control terminal or a function terminal of the power supply controller. The programmable circuit connection in the trim circuit can be programmed by applying a programming voltage to the external terminal. The shutdown circuit shuts down the power supply controller if the temperature rises above an over temperature threshold voltage. The shutdown circuit includes adjustment circuitry that can be used to test the shutdown circuit. The adjustment circuitry can adjust and reduce the over temperature threshold of the power supply controller. Thus, the power supply controller can be tested without having to actually heat the part. The disable circuit includes a programmable circuit connection, which when programmed prevents further trimming of power supply controller and prevents adjustment of the shutdown circuit over temperature threshold.

REFERENCE TO PRIOR APPLICATIONS

This application is a divisional of and claims priority to U.S.application Ser. No. 12/484,982, filed Jun. 15, 2009, now pending, whichis a divisional of U.S. application Ser. No. 12/137,453, filed Jun. 11,2008, now U.S. Pat. No. 7,710,705, which is a continuation of U.S.application Ser. No. 11/789,924, filed Apr. 25, 2007, now U.S. Pat. No.7,400,483, which is a continuation of U.S. application Ser. No.11/207,503, filed Aug. 19, 2005, now U.S. Pat. No. 7,227,733, which is acontinuation of U.S. application Ser. No. 10/834,667, filed Apr. 29,2004, now U.S. Pat. No. 6,954,057, which is a continuation of U.S.application Ser. No. 10/076,851, filed Feb. 14, 2002, now U.S. Pat. No.6,750,640, which is a divisional of U.S. application Ser. No.09/407,609, filed Sep. 28, 1999, now U.S. Pat. No. 6,388,853. U.S.application Ser. No. 12/484,982 and U.S. Pat. Nos. 7,710,705, 7,400,483,7,227,733, 6,954,057, 6,750,640, and 6,388,853 are hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power supplies and, morespecifically, the present invention relates to a switched mode powersupply controller.

2. Background Information

Electronic devices use power to operate. Switched mode power suppliesare commonly used due to their high efficiency and good outputregulation to power many of today's electronic devices. In a knownswitched mode power supply, a low frequency (e.g. 50 Hz or 60 Hz mainsfrequency), high voltage alternating current (AC) is converted to highvoltage direct current (DC) with a diode rectifier and capacitor. Thehigh voltage DC is then converted to high frequency (e.g. 30 to 300 kHz)AC, using a switched mode power supply control circuit. This highfrequency, high voltage AC is applied to a transformer to transform thevoltage, usually to a lower voltage, and to provide safety isolation.The output of the transformer is rectified to provide a regulated DCoutput, which may be used to power an electronic device. The switchedmode power supply control circuit provides usually output regulation bysensing the output controlling it in a closed loop.

A switched mode power supply may include an integrated circuit powersupply controller coupled in series with a primary winding of thetransformer. Energy is transferred to a secondary winding from theprimary winding in a manner controlled by the power supply controller toprovide the clean and steady source of power at the DC output. Thetransformer of a switched mode power supply may also include anotherwinding called a bias or feedback winding. The bias winding provides theoperating power for the power supply controller and in some cases italso provides a feedback or control signal to the power supplycontroller. In some switched mode power supplies, the feedback orcontrol signal can come through an opto-coupler from a sense circuitcoupled to the DC output. The feedback or control signal may be used tomodulate a duty cycle of a switching waveform generated by the powersupply controller or may be used to disable some of the cycles of theswitching waveform generated by the power supply controller to controlthe DC output voltage.

In order to compensate for process variations, analog integratedcircuits such as power supply controllers are commonly trimmed forcritical parameters during wafer sort, using trim pads on the waferbefore being assembled in plastic packages. Trimming is done at thewafer level because the trim pads are not usually accessible afterassembly (e.g. after encapsulation in plastic). Some of the tests arealso only done at the wafer level because they require access tointernal circuitry through test pads, which are accessible (throughprobes) only at wafer sort.

The disadvantage of trimming at wafer sort is that the trimmedparameters are subject to shifts due to physical stresses that the dieis subjected to after assembly by the encapsulation material such asplastic. This limits the accuracy to which they can be guaranteedindependent of how accurately it is trimmed at wafer sort. Consequently,the assembled parts are then tested again at final test to eliminatethose parts that have shifted too much or were damaged during assembly.

SUMMARY OF THE INVENTION

Power supply controller methods and apparatuses are disclosed. In oneembodiment, a power supply controller circuit is described including anexternal terminal and a trim circuit coupled to the external terminal.The external terminal may be any one or more of a power supply terminal,a control terminal or a function terminal of the power supply controllercircuit. The trim circuit includes a first programmable circuitconnection to be programmed in response to a signal on the externalterminal. In another embodiment, the power supply controller includes ashutdown circuit coupled to the external terminal to disable the powersupply controller during normal operation if a temperature of theshutdown circuit rises above a first threshold temperature. The shutdowncircuit includes adjustment circuitry configured to adjust the firstthreshold temperature to a second or third threshold temperature inresponse to a signal on the power supply terminal during final test andtrim of the power supply controller. In one embodiment, disablecircuitry is included to disable further testing and trimming of thepower supply controller and permanently set the shutdown circuit todisable the power supply controller when the temperature rises above thefirst temperature threshold. Additional features and benefits of thepresent invention will become apparent from the detailed description,figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a schematic illustrating one embodiment of a power supplycontroller having a trimming and testing circuitry in accordance withthe teachings of the present invention.

FIG. 2 shows one embodiment of a timing diagram illustrating a signal ona power supply terminal in accordance with the teachings of the presentinvention.

FIG. 3 is a schematic illustrating another embodiment of a power supplycontroller having a trimming and testing circuitry in accordance withthe teachings of the present invention.

DETAILED DESCRIPTION

A method and an apparatus providing final test and trimming for a powersupply controller is disclosed. In the following description, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone having ordinary skill in the art that the specific detail need notbe employed to practice the present invention. In other instances,well-known materials or methods have not been described in detail inorder to avoid obscuring the present invention.

During the fabrication process of an integrated circuit chip, the chipis often tested and trimmed. Trimming can be thought of as the processof fine-tuning or calibrating particular parameters of the chip. Theseparameters may vary from chip to chip due to a number of reasonsincluding different process variations during manufacturing. Forexample, integrated circuit reference current sources may need to betrimmed before the chips are shipped to ensure that the current sourcesprovide the proper amount of current.

Trimming can also be used to set certain programmable functions and/orparameters of the power supply controller chip such as for example butnot limited to: trimming an over voltage threshold of the power supplycontroller, trimming an under voltage threshold of the power supplycontroller, trimming switch frequencies of the power supply controller,trimming a current limit of a power switch of the power supplycontroller, and the like.

In one embodiment, the present invention allows power supply controllerparts to be trimmed during final test, after packaging,encapsulation/assembly, without using any extra electrical terminals. Inone embodiment, the trimming is done through one or more of the alreadyexisting external terminals of the power supply controller such as forexample the power supply terminal, the control terminal, the functionterminal or the like. In one embodiment, the power supply terminal is aterminal through which power supply controller circuitry is powered. Inone embodiment, the control terminal is a terminal through which thepower supply controller receives a feedback signal from the power supplyoutput. In one embodiment, the power supply terminal and the controlterminal are the same electrical terminal on the power supplycontroller. In one embodiment, the function terminal is the terminalthrough which functions of the power supply controller may becontrolled. Examples of such functions of the power supply controllercontrolled through the function terminal include but are not limited toover voltage threshold adjustment, under voltage threshold adjustment,external current limit adjustment, maximum duty cycle adjustment, powersupply enable/disable or the like.

In one embodiment, the present invention also allows internal circuitryof the power supply controller, such as a thermal shutdown circuit, tobe adjusted and tested at final test after packaging or encapsulationthrough the same V_(DD) power supply terminals. Thus, in one embodiment,an integrated circuit die including power supply controller circuitry ofthe present invention is packaged or encapsulated. Afterwards,parameters of the power supply controller may be final tested andtrimmed through an external terminal in accordance with the teachings ofthe present invention. In some instances, this may eliminate the needfor wafer sort testing all together in cases where the wafer sort yieldis high enough to make it cost effective to assemble all units on awafer and test them only once at final test to reject the defectiveparts. In addition, since trimming can be done at final test afterpackaging or encapsulation, one embodiment of the present inventionincreases the accuracy of parameters trimmed or tested because there isno shift in their value after trimming.

In one embodiment of the present invention, final test and trimming areperformed with the use of programmable circuit connections. For purposesof this disclosure, programmable circuit connections are discretionarycircuit connections that can be connected or disconnected as desired.Examples of programmable circuit connections include but are not limitedto antifuses and/or fuses. When a fuse is initially fabricated, itnormally provides a low resistance connection. After the fuse isprogrammed, the fuse provides a permanent high resistance or opencircuit connection. Conversely, when an antifuse device is initiallyfabricated, it provides a high resistance connection. However, after theantifuse is programmed, or fused, it provides a permanent low resistanceelectrical connection.

In one embodiment of the present invention, zener diodes are used forantifuse programmable circuit connections. It is appreciated, however,that fuses can be used in accordance with the teachings of the presentinvention. Unfused zener diodes are generally programmed or zapped withthe application of a high programming voltage and current. After theapplication of the programming voltage and current, the zener diode ispermanently programmed from a high resistance to a low resistanceelectrical connection.

FIG. 1 is a schematic of one embodiment of a power supply controller 101in accordance with the teachings of the present invention. Power supplycontroller 101 includes a power switch 103 coupled between a drainterminal 105 and a ground terminal. In one embodiment, power switch 103is controlled by a control circuit, shown as a pulse width modulator(PWM) 109, coupled to a control terminal and gate of power switch 103through AND gate 107. In one embodiment, final test and trim circuit 111is coupled to another input of AND gate 107.

In one embodiment, a primary winding (not shown) of a transformer of aswitched mode power supply is configured to be coupled to drain terminal105. When power switch 103 is turned on, current flows through theprimary winding of the transformer. When current flows through theprimary winding, energy is stored in the transformer. When power switch103 is turned off, current does not flow through the primary winding andthe energy stored in the transformer is transferred from a secondarywinding (not shown) to the power supply output.

In one embodiment, test and trim circuit 111 includes a selector circuit113, a disable circuit 115, a shutdown circuit 117 and one or more trimcircuits including trim circuit 119. As shown, selector circuit 113includes a hysteretic comparator 121 having an input coupled to acontrol terminal of the power supply controller 101. In one embodiment,an external power supply terminal is a control terminal and/or a V_(DD)terminal of power supply controller 101. In another embodiment,hysteretic comparator 121 can be removed and instead the input ofcounter 123 can be driven by a circuit coupled to a function terminal(not shown) of power supply controller 101. In one embodiment,hysteretic comparator 121 includes threshold voltage settings of 5.8volts and 4.8 volts. Thus, the threshold settings of hystereticcomparator 121 toggle between 5.8 volts and 4.8 volts in one embodiment.In one embodiment, counter 123 is coupled to an output of hystereticcomparator 121. Counter 123 includes a plurality of outputs includingoutputs 127, 129, 131 and 133. It is appreciated that counter 123 mayinclude more or less outputs in accordance with the teachings of thepresent invention. In the embodiment depicted, decoder 125 is coupled toreceive outputs 127, 129, 131 and 133 of counter 123. In the embodimentdepicted in FIG. 1, decoder 125 generates a plurality of outputsincluding select line 135, select line 137 and select line 141.

In one embodiment, final test and trim circuit 111 includes anothercomparator 143 having an input coupled to the power supply terminal. Inone embodiment, the threshold setting of comparator 143 is 6.2 volts. Inone embodiment, an inverter 145 is coupled to an output of comparator143.

In one embodiment, disable circuit 115 includes a programmable circuitconnection 149 coupled to the power supply terminal. In one embodiment,programmable circuit connection 149 is a zener diode or antifuse. Atransistor 151 is coupled between programmable circuit connection 149and ground. A current source 157 is also coupled between programmablecircuit connection 149 and ground. In one embodiment, current source 157is a 1 microamp current source. As shown in FIG. 1, a transistor 147 iscoupled between select line 141 and a control terminal or gate oftransistor 151. In addition, a current source 153 is coupled between thegate of transistor 151 and ground. In one embodiment, the gate oftransistor 147 is coupled to an output of inverter 145. In oneembodiment, current source 153 is a 1 microamp current source.

In one embodiment, final test and trim circuit 111 also includes aninverter 159 having an input coupled to programmable circuit connection149 and coupled to current source 157. An output of inverter 159 iscoupled to the gate of a transistor 161. An input of inverter 159 isalso coupled to a gate of a transistor 155. In one embodiment, thesource of transistor 155 is coupled to an output of inverter 145. Thesource of transistor 161 is coupled to the power supply terminal. In oneembodiment, the drain of transistor 155 is coupled to the drain thetransistor 161.

In one embodiment, shutdown circuit 117 includes current source 165,current source 167, current source 169 and current source 171 allcoupled to the power supply terminal. In one embodiment, current source169 has a current of I₁, current source 171 has a current of I₂, currentsource 167 has a current of K₁I₁ and current source 165 has a current ofK₂I₁. In one embodiment, I₂ is independent of temperature and l_(i) isproportional to temperature. In one embodiment, K₁ is a constantapproximately equal to 1.05 and K₂ is a constant approximately equal to0.3. In one embodiment, shutdown circuit 117 includes a transistor 175coupled between current source 171 and ground. In one embodiment,shutdown circuit 117 includes a resistor 177 having a resistance Rcoupled between current source 169 and ground. In addition, the base oftransistor 175 is coupled to current source 169 and resistor 177. In oneembodiment, shutdown circuit 117 includes transistor 173 coupled betweencurrent source 167 and resistor 177. In one embodiment, shutdown circuit117 also includes transistor 163 coupled between current source 165 andthe source of transistor 173. As shown in FIG. 1, the gate of transistor163 is coupled to select line 135 and the gate of transistor 173 iscoupled to programmable circuit connection 149.

In one embodiment, trim circuit 119 includes a programmable circuitconnection 181 coupled to the power supply terminal. In one embodiment,programmable circuit connection 181 is a zener diode or antifuse. Atransistor 183 is coupled between programmable circuit connection 181and ground. Trim circuit 119 also includes a reference current source191 coupled to ground. In one embodiment, transistor 187 is coupledbetween the power supply terminal and reference current source 191. Inaddition, transistor 185 is coupled between programmable circuitconnection 181 and reference current source 191. The gates oftransistors 185 and 187 are coupled to their drains and to each other.In one embodiment, trim circuit 119 also includes a transistor 179coupled between select line 137 and the gate of transistor 183. In oneembodiment, the gate of transistor 179 is coupled to the drain oftransistor 161 and coupled to the drain of transistor 155. In oneembodiment, trim circuit 119 also includes a current source 189 coupledbetween the gate of transistor 183 and ground. In one embodiment,current source 189 is a 1 microamp current source.

High level operation of final trim and test circuit 111 of FIG. 1 is asfollows. First, final test trimming of critical parameters is determinedby selecting the desired programmable circuit connections to program.For example, programmable circuit connection 181 of trim circuit 119 maybe selected for trimming during final test and trim to fine-tune thesize of transistor 187 where reference current source 191 passes. Thegate voltage of transistor 187 is used in one embodiment to generate oneor more other reference current sources (not shown) in the power supplycontroller 111. Therefore, by adjusting the size of transistor 187,these reference current sources connected to its gate are trimmed. Inone embodiment, to program programmable circuit connection 181, thevoltage on the power supply terminal is increased to a voltage wellabove the normal operating voltage. In one embodiment, the normaloperating voltage of power supply controller is 5.8V and the programmingvoltage is 11.2V. It is appreciated that other voltages may be used inaccordance with the teachings of the present invention.

In one embodiment, to select the correct programmable circuitconnection, the power supply terminal or V_(DD) is toggled between twovoltage levels. In one embodiment, the power supply terminal is toggledbetween 6.4V and 4.3V to increment counter 123. It is appreciated thatother voltages may be used in accordance with the teachings of thepresent invention. The outputs 127, 129, 131 and 133 of counter 123select a specific programmable circuit connection to be programmed. Forinstance, to select programmable circuit connection 181, V_(DD) istoggled until select line 137 selects trim circuit 119.

In one embodiment, once the desired state of the counter 123 is reached(i.e. the desired programmable circuit connection is selected), then theV_(DD) voltage is raised to the programming voltage to program theselected programmable circuit connection. In one embodiment, after anantifuse or zener diode programmable circuit connection is programmed,the programmable circuit connection becomes permanently shorted and nolonger provides a high resistance connection.

In one embodiment, after desired trimmings are completed, a finalprogrammable circuit connection is selected and programmed to preventfurther trimming of power supply controller 101. In the embodimentillustrated in FIG. 1, the final programmable circuit connection isprogrammable circuit connection 149 of disable circuit 115.

In one embodiment, each critical parameter can be trimmed to the desiredaccuracy by allocating a number of programmable circuit connections tothat parameter. When more than one programmable circuit connection isassigned to a critical parameter, each programmable circuit connectiontrim is designed to have different weight affecting that parameter.Thus, if a parameter is slightly off from the designed value, then thefirst programmable circuit connection that has the smallest weight canbe programmed. If the parameter needs more adjustment, then anothercombination of programmable circuit connections assigned to thatparameter may be programmed. Binary weighting of programmable circuitconnections can be used to reduce the number of trims for a givenaccuracy and trim range.

In one embodiment, final test trimming can also be used to alter theoperating parameters or the functionality of the power supply controller101 in a digital fashion. For example, by programming a programmablecircuit connection, an operating frequency of power supply controller101 can be halved. In one embodiment, the operating frequency may behalved after it is trimmed for accuracy.

FIG. 2 shows one embodiment of a timing diagram illustrating a signal ona power supply terminal V_(DD) for the trimming procedure forprogramming the first programmable circuit connection and the finalprogrammable circuit connection. In the example illustrated in FIG. 2,assume that programmable circuit connection 181 of FIG. 1 is selected inthe S1 state and that programmable circuit connection 149 is selected inthe S15 state.

As shown in the embodiment depicted, the power supply terminal V_(DD) istoggled between 6.4V and 4.3V to increment counter 123. Indeed, 6.4V isgreater than the upper threshold (5.8V) of hysteretic comparator 121 and4.3 volts is less than the lower threshold (4.8V) of hystereticcomparator 121. It is appreciated that other voltages may be used inaccordance with the teachings of the present invention. As will bediscussed in greater detail below, in the S1 state, select line 137selects programmable circuit connection 181 and the application of 11.2volts programs programmable circuit connection 181. Similarly, in theS15 state, select line 141 selects programmable circuit connection 149and the application of 11.2 volts programs the final programmablecircuit connection 149. In one embodiment, after the programmablecircuit connection 149 is programmed, further trimming by programmingany of the unprogrammed programmable circuit connections is notpossible, which avoids accidental trimming in the future.

In one embodiment, shutdown circuit 117 provides self protection topower supply controller 101 with over temperature shutdown. In oneembodiment, if the internal temperature of the power supply controller101 reaches the over temperature threshold, power switch 103 is nolonger switched and the power supply is thereby disabled. In oneembodiment, the over temperature threshold is approximately 135° C.

In one embodiment, over temperature threshold testing during final testis done at room temperature, before final programmable circuitconnection 149 is programmed. In one embodiment, before programmablecircuit connection 149 is programmed, shutdown circuitry 117 operates intwo modes. In the first mode, an addition of one mirrored current source167 adjusts or reduces the thermal shutdown threshold to typically 33°C. in one embodiment. In the second mode, which can be selected bysetting the counter 123 to the particular state to activate select line135, another current source 165 is added. In one embodiment, the secondmode can be alternatively selected through another external terminal(not shown). In one embodiment, this further adjusts or reduces thethermal shutdown temperature threshold to 13° C.

In one embodiment, when testing the power supply controller 101 in thesetwo modes at room temperature (e.g. 22° C.), power supply controller 101will not be in thermal shutdown in mode one. In mode two, however, powersupply controller 101 should be in thermal shutdown mode. Therefore, inone embodiment, the over temperature aspect shutdown circuitry 117 canbe tested without actually having to raise the temperature of powersupply controller 101 to the thermal shutdown temperature (e.g.approximately 135° C.). In one embodiment, these modes of operation willbe disabled as soon as programmable circuit connection 149 isprogrammed. In one embodiment, the testing of two modes beforeprogrammable circuit connection 149 is programmed verifies that thethermal shutdown threshold afterwards will be about 135° C. with avariation of +/−10° C.

A detailed description of the final test and trim circuit 111 of FIG. 1is as follows. As mentioned above, the power supply terminal V_(DD) istoggled in one embodiment between 6.4V and 4.3V to select the desiredprogrammable circuit connections. The output of hysteretic comparator121 is received by counter 123. In one embodiment, counter 123 is aresetable flip-flop counter, which is reset during initial power up ofpower supply controller 101. The outputs 127, 129, 131 and 133 of thecounter 123 are received by decoder 125, which selects the desiredprogrammable circuit connections to be programmed. In one embodimentcounter has N outputs and decoder has 2^(N) select lines as outputs.

In the embodiment depicted in FIG. 1, to select programmable circuitconnection 181, V_(DD) is first raised to 6.4V then lowered to 4.3V andthen raised again to 6.4V. In FIG. 2, this corresponds to state S1. Atthis point, the decoder 125 activates signal line 137. In oneembodiment, comparator 143 is a comparator with a threshold of 6.2V.When V_(DD) is above 6.4V, the output of comparator 143 is high and theoutput of inverter 145 is low. Transistor 155 is on and transistor 161is off since programmable circuit connection 149 is not programmed yet(high impedance) and the gate of transistor 155 is pulled low by currentsource 157. Since transistor 155 is on, the low signal at the output ofinverter 145 appears at the gate of transistor 179, turning it on. Thegate of transistor 183 is now connected to the select signal line 137,which was activated. Transistor 183 therefore turns on, enablingprogrammable circuit connection 181 to be programmed.

In one embodiment, however, transistor 183 can not yet programprogrammable circuit connection 181 because programmable circuitconnection 181 requires about 40 mA of current flowing through it. Inone embodiment, this requires a power supply terminal V_(DD) voltage of10V since the unprogrammed zener voltage of programmable circuitconnection 181 is its zener junction voltage drop plus the resistivedrop when the required 40 mA is applied. In one embodiment, the Zenerjunction voltage drop is 4.7V. In one embodiment, the resistive drop isalmost 4V, which translates into approximately 40 mA times the zenerresistance of programmable circuit connection 181, which isapproximately 100 Ohms. Transistor 183 is designed such that the voltagefrom drain to source is small when it passes 40 mA. In one embodiment,in order to proceed with programming programmable circuit connection181, V_(DD) must therefore be raised to about 11.2V. When programmablecircuit connection 181 is programmed, the source of transistor 185 isthereby shorted to V_(DD) and transistors 185 and 187 are connected inparallel to V_(DD), thereby increasing the effective size of transistor187. In this example, by increasing the size of transistor 187, thetrimming of a certain critical parameter is achieved.

FIG. 3 is a schematic illustrating another embodiment of a power supplycontroller having trimming and testing circuitry in accordance with theteachings of the present invention. FIG. 3 shows final test trimming togenerate a digital level digital output 315 that can be used to selectfeatures or change parameters in a digital fashion (such as a resistorusing switches driven by digital output 315 to short out parts of it).As mentioned earlier, final test trimming in one embodiment may be usedfor trimming an over voltage threshold of the power supply controller,trimming an under voltage threshold of the power supply controller,trimming switching frequencies of the power supply controller, trimminga current limit of a power switch of the power supply controller, or thelike.

In one embodiment, the schematic of FIG. 3 is substantially similar tothe schematic of FIG. 1 with the exception of trim circuit 319 beingillustrated. In one embodiment, trim circuit 319 can be included in thesame power supply controller 101 as trim circuit 119. In such anembodiment, trim circuits 119 and 319 would be individually selected bydifferent select lines (e.g. 137, 339) from decoder 125.

In one embodiment, trim circuit 319 includes a programmable circuitconnection 303 coupled to the power supply terminal. In one embodiment,trim circuit 319 also includes a transistor 307 coupled between thepower supply terminal a reference current source (not shown) throughwhich a reference current I_(REF) 317 flows. In one embodiment, the gateof transistor 307 is coupled to the drain of transistor 307. In oneembodiment, a transistor 305 is coupled between programmable circuitconnection 303 and digital output 315 of trim circuit 319. The gate oftransistor 305 is coupled to the gate and drain of transistor 307. Inone embodiment, a current source 313 is coupled between digital output315 and ground. In one embodiment, current source 313 is a 1 microampcurrent source. Trim circuit 319 also includes a transistor 309 coupledbetween programmable circuit connection 303 and ground. In oneembodiment, trim circuit 319 includes a current source 311 coupledbetween the gate of transistor 309 and ground. In one embodiment,current source 311 is a 1 microamp current source. In one embodiment,trim circuit 319 also includes a transistor 301 coupled between selectline 339 and the gate of transistor 309. In one embodiment, the gate oftransistor 301 is coupled to the drain of transistor 161 and the drainof transistor 155.

In one embodiment, if programmable circuit connection 303 is notprogrammed (i.e. not short circuited), then current source 313 pullsdigital output 315 low. In one embodiment, if programmable circuitconnection 303 is programmed (i.e. short circuited), then 10 microampsof current could flow through the current mirror formed with transistor305, which would overpower the 1 microamp of current source 313, whichwould pull the digital output 315 signal high.

Similarly other programmable circuit connections that are desired to beprogrammed can be first selected by toggling counter 123 to theappropriate state and programming the programmable circuit connectionsas described above. In one embodiment, after all the requiredprogrammable circuit connections are programmed, the status of allprogrammable circuit connections can be confirmed or verified bytoggling counter 123 to each state and increasing the power supplyV_(DD) voltage to 8V at each state while recording the V_(DD) current(the supply current to power supply controller 101). For the states thatpoint to the programmable circuit connections that are programmed, theV_(DD) current is significantly higher than those states which point tothe programmable circuit connections that were not programmed (about 50mA in one embodiment).

Referring for example to trim circuit 119 of FIG. 1, the additionalcurrent is due to transistor 183 turning on. During the state in whichselect line 137 is active, if V_(DD) is raised to for example 8V, theoutput of inverter 145 goes low, transistor 179 turns on, turningtransistor 183 on. Since programmable circuit connection 181 wasprogrammed, the drain of transistor 183 goes close to 8V and transistor183 conducts as much current as its physical size allows. Hadprogrammable circuit connection 181 not been programmed, the voltage atthe drain of transistor 183 would be lower. The voltage drop across thezener junction and the respective resistive drop across the zener wouldlimit the drain voltage of transistor 183 and the current would be muchlower.

In another embodiment, programmable circuit connections that areprogrammed can be verified immediately after programming. In particular,the V_(DD) supply current at 8V may be measured before programming theprogrammable circuit connection at 11.2V and then rechecked again at 8Vto confirm that it is higher by the expected amount (e.g. 50 mA in oneembodiment). If the programmable circuit connection was not programmed(e.g. no change in current) or not programmed properly (e.g.insufficient change in current) it can be reprogrammed. In oneembodiment, this process can be repeated several times until a properprogramming is achieved.

In one embodiment, after programming all of the desired programmablecircuit connections, the final programmable circuit connection 149 ofdisable circuit 115 can be programmed to prevent any further accidentalprogramming of the programmable circuit connections during normaloperation of the power supply controller 101. Referring to theembodiment illustrated in FIG. 1, in order to program programmablecircuit connection 115, the toggling of V_(DD) between 6.4V and 4.3Vshould continue as shown on FIG. 2 until the S15 state and select line141 is thereby activated by decoder 125. When programmable circuitconnection 149 is programmed, the gate of transistor 155 is pulled highto V_(DD) since programmable circuit connection 149 is short circuited.Since the gate of transistor 155 is high, transistor 155 turns off andtransistor 161 turns on through inverter 159, pulling the gate oftransistor 179 high, turning transistor 179 off permanently. Thus, anyfurther trimming is not possible after programmable circuit connection149 is programmed or short circuited.

Considering another power supply controller 101 where only programmablecircuit connection 149 of disable circuit 115 is programmed, even ifV_(DD) is toggled again from the start in an attempt to program forexample programmable circuit connection 181, select line 137 would gohigh, but since transistor 179 is permanently off, the gate oftransistor 183 would never go high and transistor 183 would always stayoff. Hence, transistor 183 can no longer program programmable circuitconnection 181.

With regard to shutdown circuit 117, in one embodiment, over temperaturethreshold testing can be done at final test. As discussed, shutdowncircuitry in one embodiment includes current source 169, current source171, transistor 175 and resistor 177. In one embodiment, shutdowncircuitry also includes adjustment circuitry including transistor 163,transistor 173, current source 167 and current source 165, which may beused for over-temperature threshold testing at final test.

In one embodiment, current source 169 generates a current l_(i)proportional to the thermal voltage V_(T). In one embodiment, the valueof the voltage I₁R across resistor 177 at room temperature isapproximately 300 mV. With the V_(T) variation (KT/q) acrosstemperature, this voltage increases at a rate of 1 mV/° C. The V_(BE) ofthe transistor 175 with collector current of I₂ from current source 171is typically 650 mV at room temperature. It decreases at a rate of 2.2mV/° C. In one embodiment, the current I₂ supplied by current source 171is constant across temperature.

In one embodiment, during the final test, before programmable circuitconnection 149 is programmed, the gate of transistor 173 is pulled lowand transistor 173 is on. When counter 123 is toggled to the S15 stateand select line 141 is activated, the gate of transistor 163 is highand, therefore, transistor 163 is off. The voltage across resistor 177is then: R×(K₁I₁+I₁). This voltage across resistor 177 is designed to belower than 650 mV at room temperature and is therefore not high enoughto turn on transistor 175. Thus, over-temperature shutdown is nottriggered and power switch 103 keeps on switching.

In one embodiment, when the S15 state is deselected and select signal151 is deactivated, the gate of transistor 163 is low turning transistor163 on. Thus, the voltage across resistor 177 increases to:R×(K₂I₁+K₁I₁+I₁). In one embodiment, this voltage across resistor 177 isnow higher than turn on threshold voltage of transistor 175 at roomtemperature. As a result, transistor 175 turns on, pulling the collectorof transistor 175 low and triggering an over-temperature shutdown. Whenthe collector of transistor 175 goes low, the gate of the power switch103 goes low through AND gate 107 and disables power switch 103 fromswitching.

In one embodiment, after programmable circuit connection 149 isprogrammed, the gate of transistor 173 is pulled high and transistor 173permanently turns off, disconnecting the current sources 165 and 167from resistor 177. Therefore, the over-temperature threshold of shutdowncircuit 117 will no longer be adjusted and the over-temperaturethreshold will not be reached until the voltage across resistor 177,I₁×R, exceeds the V_(BE) of transistor 175, which in one embodiment isapproximately 135° C.

The governing equations of one embodiment of shutdown circuit 117 arelisted below:

The thermal shutdown temperature of shutdown circuit 117 with K₁approximately equal to 1.05 and K₂ approximately equal to 0.3 is:

[(650−I ₁ R)/(2.2+1)]+25° C.=134.4° C.;

the final testing low threshold is:

{[650−(1+K ₁ +K ₂)I ₁ R]/[2.2+(1+K ₁ +K ₂)]}+25° C.=12.9° C.;

the final testing high threshold is:

{[650−(1+K ₁)I ₁ R]/[2.2+(1+K ₁)]}+25° C.=33.2° C.;

the lowest threshold is:

[K ₁ I ₁ R/(2.2+1)]+25° C.=123.4° C.;

the highest threshold is:

[(K ₁ +K ₂)I ₁ R/(2.2+1)]+25° C.=151.6° C.

In the foregoing detailed description, the method and apparatus of thepresent invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A method, comprising: selecting a trim circuit of an integrated circuit with a first external function terminal of the integrated circuit; applying a programming voltage to a second external terminal after selecting the trim circuit to program a first programmable circuit connection of the trim circuit, wherein the second external terminal is one of a supply terminal, a control terminal and a second function terminal of the integrated circuit; selecting a disable circuit of the power supply controller circuit; and applying the programming voltage to the second external terminal after selecting the disable circuit to program a second programmable circuit connection of the disable circuit, the disable circuit to disable the trim circuit of the integrated circuit from being further programmed.
 2. The method of claim 1, wherein the integrated circuit is configured to operate with an operating voltage at the second external terminal and wherein the programming voltage is greater than the operating voltage.
 3. The method of claim 2, wherein the operating voltage is a non-zero operating voltage.
 4. The method claim 1, wherein the trim circuit is one of a plurality of trim circuits and wherein selecting the trim circuit of the integrated circuit with the first external function terminal of the integrated circuit comprises selecting one of the plurality of trim circuits with the first external function terminal.
 5. The method of claim 1, wherein integrated circuit is a power supply controller. 